In the state of the art noble metal-containing oxidation catalysts for exhaust gas purification systems in both stationary and mobile applications are known. Oxides or oxide mixtures selected from Al—, Ti—, Ce—, La, Zr—, Sn—, W—, Y—, Pr—, Gd oxides and optionally further alkaline-earth oxides are normally used as active carrier substance. These oxides are usually applied as washcoat to ceramic or metal substrates (e.g. honeycomb bodies) and then impregnated with a noble metal solution. Alternatively the noble metal components can be applied to one or more oxides, fixed by calcining and then applied to the carrier as catalytically active washcoat. This is referred to as a “one-step process”. Noble metals which are used in oxidation catalyst's are frequently Pt, Pd, Au, Ag, Rh, Re, Ir, wherein these noble metals are usually present as metal clusters.
In addition, the redox-active transition metals Mn, Fe and Cu are also frequently used in oxidation catalysts.
In the state of the art it is disadvantageous, inter alia, that in the course of their use, the metal clusters lose their optimum activity, determined by an optimum cluster size, due to ageing. In other words, due to sintering of the metal clusters of optimum size, larger clusters with reduced active surface area form. The optimum size of the active metal clusters is normally clearly smaller than the average pore size of the washcoat, which is why the metal clusters have enough room to grow onto the larger, less active clusters above a specific temperature.
Ageing can however also take place due to a reduction of the accessible surface area of the washcoat, for example by conversion of the large-surface γ-aluminium oxide to small-surface α-aluminium oxide. This reduces the accessibility for the reaction gases to the surface area and catalyst activity decreases.
Deactivation of the catalyst by poisoning, e.g. by sulphur, SiO2 or other catalyst poisons is also known.
In the state of the art zeolites already coated with noble metal were therefore used in order to reduce temperature-related damage. Although zeolites form very stable structures, they can be damaged at high temperatures and in particular by the action of steam (e.g. by dealumination), which leads to a reduction of their inner surface area and involves a reduction in activity.
A further disadvantage with zeolites is that they usually have Brønsted acid centres which negatively affect the stability of the active metal clusters of oxidation state 0 which possess the highest activity for many oxidation reactions.
It would therefore be advantageous to use alternative compounds which are stable at high temperatures and prevent the metals from forming clusters.